POSUM (Parallel Oregon State University Model)

Modeling the annual cycle of wind-driven circulation of the Pacific Ocean is the first application of a new, isopycnic-coordinate ocean general circulation model developed recently for climate studies. In the present experiment, the model domain features realistic bottom topography and coastlines but is limited by artificial, closed boundaries at 62° and 38°S. Initial stratification is represented by ten flat isopycnal layers with layer volumes determined from Levitus data integrated between the corresponding isopycnals. In response to seasonal forcing by COADS winds, the model develops realistic stratification and circulation after three decades of integration. The time-averaged output reproduces important features of the stratification of the Pacific Ocean such as the zonal slope of isopycnals along the equator and the bowl shape of the subtropical gyres. Beneath the gyres, regions of homogenized potential vorticity form. Layers are permitted to outcrop at the surface, but the absence of a mixed layer in adiabatic runs means that the sea surface distribution of density is not realistic. Annual variability in the model is analyzed in terms of simple planetary wave theory. These waves demonstrate behavior recently observed by the TOPEX mission, including increased phase speeds and amplification in the western North Pacific.